224 research outputs found
H-alpha features with hot onsets. II. A contrail fibril
The solar chromosphere observed in H-alpha consists mostly of narrow fibrils.
The longest typically originate in network or plage and arch far over adjacent
internetwork. We use data from multiple telescopes to analyze one well-observed
example in a quiet area. It resulted from the earlier passage of an
accelerating disturbance in which the gas was heated to high temperature as in
the spicule-II phenomenon. After this passage a dark H-Halpha fibril appeared
as a contrail. We use Saha-Boltzmann extinction estimation to gauge the onset
and subsequent visibilities in various diagnostics and conclude that such
H-alpha fibrils can indeed be contrail phenomena, not indicative of the
thermodynamic and magnetic environment when they are observed but of more
dynamic happenings before. They do not connect across internetwork cells but
represent launch tracks of heating events and chart magnetic field during
launch, not at present.Comment: Accepted for Astronomy & Astrophysic
On the multi-threaded nature of solar spicules
A dominant constituent in the dynamic chromosphere are spicules. Spicules at
the limb appear as relatively small and dynamic jets that are observed to
everywhere stick out. Many papers emphasize the important role spicules might
play in the energy and mass balance of the chromosphere and corona. However,
many aspects of spicules remain a mystery. In this Letter we shed more light on
the multi-threaded nature of spicules and their torsional component. We use
high spatial, spectral and temporal resolution observations from the Swedish
1-m Solar Telescope in the H{\alpha} spectral line. The data targets the limb
and we extract spectra from spicules far out from the limb to reduce the
line-of-sight superposition effect. We discover that many spicules display very
asymmetric spectra with some even showing multiple peaks. To quantify this
asymmetry we use a double Gaussian fitting procedure and find an average
velocity difference between the single Gaussian components to be between 20-30
km s for a sample of 57 spicules. We observe that spicules show
significant sub-structure where one spicule consists of many 'threads'. We
interpret the asymmetric spectra as line-of-sight superposition of threads in
one spicule and therefore have a measure for a perpendicular flow inside
spicules which will be important for future numerical model to reproduce. In
addition we show examples of {\lambda}-x-slices perpendicular across spicules
and find spectral tilts in individual threads providing further evidence for
the complex dynamical nature of spicules.Comment: Accepted by APJ Letter
Small-scale swirl events in the quiet Sun chromosphere
Recent progress in instrumentation enables solar observations with high
resolution simultaneously in the spatial, temporal, and spectral domains. We
use such high-resolution observations to study small-scale structures and
dynamics in the chromosphere of the quiet Sun. We analyze time series of
spectral scans through the Ca II 854.2nm spectral line obtained with the CRISP
instrument at the Swedish 1-m Solar Telescope. The targets are quiet Sun
regions inside coronal holes close to disc-centre. The line core maps exhibit
relatively few fibrils compared to what is normally observed in quiet Sun
regions outside coronal holes. The time series show a chaotic and dynamic scene
that includes spatially confined "swirl" events. These events feature dark and
bright rotating patches, which can consist of arcs, spiral arms, rings or ring
fragments. The width of the fragments typically appears to be on the order of
only 0.2", which is close to the effective spatial resolution. They exhibit
Doppler shifts of -2 to -4 km/s but sometimes up to -7 km/s, indicating fast
upflows. The diameter of a swirl is usually of the order of 2". At the location
of these swirls, the line wing and wide-band maps show close groups of
photospheric bright points that move with respect to each other. A likely
explanation is that the relative motion of the bright points twists the
associated magnetic field in the chromosphere above. Plasma or propagating
waves may then spiral upwards guided by the magnetic flux structure, thereby
producing the observed intensity signature of Doppler-shifted ring fragments.Comment: 4 pages, 3 figures, A&A Letter, accepted (final version
Surges and Si IV bursts in the solar atmosphere. Understanding IRIS and SST observations through RMHD experiments
Surges often appear as a result of the emergence of magnetized plasma from
the solar interior. Traditionally, they are observed in chromospheric lines
such as H 6563 \AA and Ca II 8542 \AA. However, whether there is a
response to the surge appearance and evolution in the Si IV lines or, in fact,
in many other transition region lines has not been studied. In this paper we
analyze a simultaneous episode of an H surge and a Si IV burst that
occurred on 2016 September 03 in active region AR12585. To that end, we use
coordinated observations from the Interface Region Imaging Spectrograph (IRIS)
and the Swedish 1-m Solar Telescope (SST). For the first time, we report
emission of Si IV within the surge, finding profiles that are brighter and
broader than the average. Furthermore, the brightest Si IV patches within the
domain of the surge are located mainly near its footpoints. To understand the
relation between the surges and the emission in transition region lines like Si
IV, we have carried out 2.5D radiative MHD (RMHD) experiments of magnetic flux
emergence episodes using the Bifrost code and including the non-equilibrium
ionization of silicon. Through spectral synthesis we explain several features
of the observations. We show that the presence of Si IV emission patches within
the surge, their location near the surge footpoints and various observed
spectral features are a natural consequence of the emergence of magnetized
plasma from the interior to the atmosphere and the ensuing reconnection
processes.Comment: 13 pages, 8 figures. The Astrophysical Journal (Accepted
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